I play support cleric as my main. I've played in many kinds of groups. Some groups are really good with high amount of dps and good teamwork. Some group are average.
But what really bugs me the most are dpsers that don't know how to assist another and just go off wandering solo and over-extending.
In the end, everyone is on there own, all scattered through the whole map. It makes getting a kill harder and also makes effective group healing for me waaay harder. Proper focus can change the whole game for you team. So for all you solo dpsers, you better read this tutorial carefully.
Why assisting/focusing?
In general, when 2 groups are going to clash, the group with the best focus wins.
Let's compare focusing with a Real Time Strategy game. Take Red Alert 2 for example. I'd like to add that this is just theory, just to give you a picture why focusing is so important.
Group Red and Blue has both 5 Rhino tanks ready for battle. General Red is a player that knows the concept of focusing. General Blue has no clue about that whatsover and normally let his army decide on their own who to attack. When their Rhino Tanks are going to clash, this is how it'll look like:
Let's assume that 1 Rhino Tank has 5 HP and does 1 damage per hit and all these tanks shoot all at the same time.
The red/blue lines indicates wich target they're firing at.

As you can see, the Red General wins due to micro managing focusing targets.
Communication and teamwork
These are probably the most important aspects in group PvP.
Problem with PvPing with randoms is that you can't communicate as well as with a mic through TeamSpeak/Skype etc. But that doesn't mean that you can't have a good teamwork.
Before actaully going out in battle with your group, you need to make sure that your group knows the plan. 1 of those plans is to decide wich player picks the target and who is going to assist him/her. When you group play according to the plan, it can change the gameplay drastically in your favor.
How do I assist a target?
First of all, the ~ button (wich is next to the number 1 key) is the default assist button. What this function does is it'll select the target of your current selected target.
Sounds too complicated? Let me give you an example:

As you can see here, I am selecting Folly (my target) atm and Folly is selecting Shki (my target's target). If I press the ~ right now, I will switch to my target's target, making me selecting Shki.
Now that you know how the ~ assist button works, let's continue about actually assisting in PvP as a dpser.
What you need to do before going into battle is managing your group list window. You're not a support priest or something so you don't need to go re-arrange every group member in your list. You only have to put the player you're going to assist on top of the list. This will make you select that player when you press F2, IF you use default function for F2. If not, you can also make a macro with ~ as bind key. The macro should look like this:
/select [Group1]
/Skill Select Target's Target
Note that you need to type down the capital letters too, otherwise it won't work.
Conclusion
Assist your team mate and you'll do fine.